The present invention relates generally to firing circuits for thyristor power conversion systems and more particularly to an improved circuit for controlling the operation of the power conversion system which supplies an electrical load from a polyphase alternating current (a.c.) source.
There are many circuits and systems available today for selectively rendering conductive the controlled rectifiers of the various types of converters for supplying electrical power to a load from a polyphase a.c. source. The type of rectifier used will, of course, control to some degree the type of control utilized but by far the most common controlled rectifier in use today is a thyristor of the silicon controlled rectifier type. The thyristor becomes conductive with the simultaneous application of a forward bias voltage and a signal applied to its gate electrode and remains conductive until a zero or negative voltage is placed across its anode and cathode.
A large variety of problems exist in the control of power converters. Included among these problems is the fact that, because the magnitude of the gate signal required to render a thyristor conductive is not large, adequate protection from electrical noise, both line and otherwise, must be provided to prevent a erroneous thyristor firing. In addition, the ability to alter and optimize firing sequence, particularly in a reversing converter, is very desirable in order to provide smooth, continuous system operation. Further, it is necessary to provide means for recovery of the system in the event a thyristor for some reason is conducting at an improper time. The most common reason for improper conduction is that a thyristor fails to commutate (turn off) at the proper time and the customary correction method is to fire the next scheduled to conduct thyristor at an earlier than normal time to force the erroneously conducting thyristor into a nonconducting state. This is commonly called "force firing". A further problem which is particularly prevelant in polyphase converter circuits is the need to synchronize the thyristor firing properly in each phase. This is customarily achieved by closely matching each phase firing circuit to obtain uniform action. Component aging and temperature drift render maintenance of proper synchronization and matching difficult.
All of the above problems are recognized in the art and many solutions have been proposed, often at a relatively high cost. Usually a compromise is reached between cost and performance.